Trials Riding technical infoUCI rules (Union Cycliste Internationale): (no bashguard or pedal contact with the obstacles)..
BIU rules (BikeTrial International Union): (bashguard and pedal support allowed)...
NATS rules (North American Trials Series): (bashguard and pedal support allowed)...
Trials Online: Huge international forum on trials riding, plenty of FAQs, tips, bikes and components info etc.. and loads of moves in videos (.mpg) if you can browse them.
Trials Forum: One of the most active forum about trials riding in the UK..
Observed Trials: Excellent Trials forum in the US
BikeTrials: One of the most complete sites on trials riding. (history, FAQs, rules, advice on bikes, links to loads of trials websites.
Trial Spain: An excellent spanish website with plenty of good pics and info about trials.
Biketrial-Spain: Extensive spanish site about trials, both in Spanish and English.
UCI: Union Cycliste Internationale: the official site for all infos about biking events worldwide
Biketrial UK: The official UK biketrials organisation (calendars, comps, etc..)
Trials World: forum site with some pics and plenty of good videos.
Koxx Training Videos: 10 short videos that show all the basic moves to start with.
Trials Queens: there are some girls riding trials out there.
ProPatcher: A complete tutorial about trials riding techniques (in text mainly, but very detailed).
Velo Trial Club Zurich: Very good site with plenty of news and videos (how-to section in german).
Hans Ray's website: One of the pioneers of bike trials.
Neil Tunnicliffe's website: Some mental riding going on there... you urban legend
Craig Lee Scott's website: Mind boggling videos, mental dexterity and control for urban trials.
Mike Trials: a site from Mike Bentham, pretty damn good rider, selling videos.
Ryan Leech's website: Excellent videos, with plenty of cool moves.
Streetfurniture Trials: as it says on the tin... Trials pics and videos, mainly in urban settings.
Trials works: a site with quite a few videos.
canberratrials: a site with plenty of trials short mpeg videos to see, and loads of crash examples.
The M.A.D team: organises trials demos and mountainbike stunts.
ThinkBikes: again, promoting trials through demos around the UK.
RJ Ride Guide: biking safety promoted through trials.
Engoedidee: Plenty of videos of extreme sports, by category
Zerodc: Dirt mountain biking site, in french.
The Biggest Links Page: the most complete listing of trials websites i've ever seen (by country)
The Magura Cult page: all you want to know about Magura brakes

Other bicycle-related info or tutorial websitesHennigar: Freeride Video game developer.
Sheldon Brown: The largest lexis about bicycle parts and plenty of tips to fix and maintain your bike.
International Bicycling Fund: Promoting cycling as a sustainable mean of transport.
Why cycling?: basic answers to promote cycling.
Bicycle Mojo: Excellent site for old or vintage bicycle repairs and hard to find parts.
BikeForum.net with plenty of cycling categories.
Gravity Bike weird bikes with no cranks, for pure road downhill.
Ice Bike: All you need to know about cycling in freezing conditions, without becoming frozen.
Bikerod & Kustom: Only custom bikes, and gallery of homemade bikes.
Cyclery Hub plenty of links to other bike manufacturers and bike parts.
Recumbents: if you want to know more about these beasty bikes, here is the source.
Inspired Cycles Engineering: Trices or threecycle kart-type recumbents.
KMX Kart cycles: a sports approach to cycling on three wheels.